The output of logic circuits utilizing Josephson devices is usually a small current developed as a result of the switching of one or more of the Josephson devices. This current or absence of the current designates a logic 1 or 0, respectively. The detection of this small current is often difficult because of the current that already exists in the logic line, such as gate current, from which the small current change must be distinguished. This existing current can be considered as noise and thus the signal-to-noise ratio is small.
Josephson devices are superconductive devices where superconductivity means the complete disappearance of the electrical resistivity of a number of metals and alloys at very low temperatures near absolute zero. Superconductivity occurs suddenly below a certain temperature. This so called critical temperature differs from material to material. It is known that electrons may be driven across a potential barrier between two conductors separated by a thin, non-conducting layer. The barrier is crossed by tunnelling. Hence, the effect is called "the tunnelling effect". When the conductors are superconductive, single electrons may tunnel through such areas, whereby they overcome a potential difference corresponding to the value of the gap voltage. However, besides this single electron tunnelling effect there exists a further superconductive tunnelling effect involving bound electron pairs. In a superconductive metal, the prevailing charge carriers involved are electron pairs coupled with the lattice by the electron phonon interaction. Such electron pairs are called "Cooper pairs" after the scientist of that name. It has been predicted by B. D. Josephson, that a tunnelling effect involving Cooper pairs as carriers should occur between two adjacent superconductive metals separated by a sufficiently thin insulating layer. The insulating layer of material behaves like a superconductive metal in this Josephson tunnelling effect. With this kind of tunnelling, no potential difference is passed and, accordingly, no resistance is encountered by the current. The basic paper covering the Josephson tunnelling effect is the "Possible New Effects In Superconductive Tunnelling," published by B. D. Josephson in issue No. 7, Vol. No. 1 of the periodic Physic Letters, dated July 1, 1962 on pages 251 through 253. Generally the Josephson element or junction consists of a thin oxide barrier layer between two superconductive leads. At least one control line is provided for controlling the switching behavior of the element. The superconductive Josephson elements operate at a temperature of a few degrees kelvin and can take two different states depending on the passing current. In the region below the maximum Josephson current Imax, a Cooper pair tunnelling current is flowing. This is called "pair tunnelling," and the voltage drop across the element is zero. The element is in the superconductive state. When the current value Imax is exceeded the element switches to the other state, the finite voltage state. The current obtained is a tunnelling current which, however, involves essentially single charge carriers or quasi-particles. The particle tunnelling process is accompanied by a voltage drop caused by the tunnelling current, which voltage drop corresponds to the energy gap voltage Vg.
In the so-called finite voltage state of a Josephson element, the leads remain in the superconducting state, and a particle tunnelling current flows across the isolating interface. In Josephson elements a very small voltage drop occurs, which results in very little heat dissipation. When the current is reduced or removed after switching into the finite voltage state, a hysteresis effect occurs, i.e. resetting by changing to the superconducting state occurs at a current value considerably below the value Imax. The maximum Josephson current value Imax can be influenced by controlling magnetic fields applied by the current passing through the adjacent control lines. The maximum Josephson current value at which switching from the no voltage or superconducting state to the finite voltage state occurs can be varied by the design of the Josephson device.
Since Josephson elements can take two distinguishable states, they can be utilized in binary circuits. In U.S. Pat. No. 3,281,609, a superconductive switching element utilizing tunnelling effects is described. Input currents to be connected are applied through a Josephson junction. If the current flowing through the Josephson junction overcomes the maximum Josephson value, the junction switches to the finite voltage state. The resulting output current flows through a load resistance connected in parallel to the Josephson junction. U.S. Pat. No. 3,626,391 is an example of the application of Josephson elements to a memory application. The binary values are represented by the direction of circulating current in superconducting loops. Switching the current direction and sensing the storage value is effected by Josephson elements.
A paper by J. Matisoo, "The Tunnelling Cryotron -- A Superconductive Logic Element Based on Electron Tunnelling" appears in the February 1967 issue of the Proceedings of the IEEE, issue No. 2, Vol. 55. This paper describes an application of Josephson elements as logic gates. It is concerned essentially with the switching behavior of a single element. It is suggested in one of the figures, that the current distribution in two branches of a superconducting loop can be controlled.
In U.S. Pat. No. 3,758,795 a binary logic circuit has been disclosed which utilizes Josephson elements, whereby, a line terminated by its characteristic impedance is connected in parallel to a Josephson junction. In the superconducting state, the current flows essentially through the Josephson junction, but in the normal conducting state it flows through the parallel line. Subsequent elements can be controlled by the current in the line. These superconductive circuits are sensing whether the gate current is present or not. They are not concerned with a current change in addition to or subsequent with respect to the gate current.
Even with the knowledge of these superconductive Josephson tunnelling effects and their application to logic circuits and switching devices, it was not readily apparent how the Josephson tunnelling effect could be applied to reliable sensing circuits, especially one that can provide sensing with a high signal-to-noise ratio even though the signal must be detected in the presence of the Josephson gating signal.
Accordingly, it is the main object of the present invention to provide a sensing circuit for use in logic circuits utilizing superconductive Josephson tunnelling devices to provide a sensing signal having an improved signal-to-noise ratio.
It is another object of the present invention to provide a sensing circuit which isolates the current to be detected from the so called noise current so that the current to be detected has a theoretical infinite signal-to-noise ratio.
It is a further object of the present invention to provide a sensing circuit which utilizes a superconductive Josephson tunnelling device as the main element thereof.